The selective catalytic reduction (SCR) of nitrogen oxides produced in combustion engines, with reductants such as urea and ammonia, is an industrially significant catalytic process. Vanadia-based SCR catalysts that utilize titania catalyst supports are approved for use for on-road mobile applications in Europe on Heavy-Duty Diesel trucks, and these catalyst are highly active and show excellent tolerance to fuels that contain sulfur. However, vanadia-based catalysts are not approved by the EPA for on-road use in the U.S. or in Japan. This lack of approval stems from the concern over release of vanadia into the environment and the potential toxicity that might arise from exposure to vanadia emitted from the tailpipe. One possible mechanism that potentially might cause a loss of vanadia from the catalyst is vaporization of the metal oxide or hydroxide at high temperature in the stream of hot exhaust gases.
Furthermore, newer regulations for soot and NOx that will begin to be imposed as early as 2010 (e.g., Euro VI and US 2010 regulations) may necessitate the use of both a diesel particulate filter (DPF) in tandem with an SCR catalyst. In one configuration (U.S. Pat. No. 7,498,010) the SCR catalyst is downstream from the DPF. If no remedial action is taken, the collection of soot on the DPF, will ultimately plug the channels for exhaust gas flow, and can cause unacceptable pressure-drop arises across the device. In order to avoid this situation, the soot is removed either continuously or sporadically by combustion. Since combustion is an exothermic process, it is associated with a rise in the temperature of the device that is transmitted to the exhaust gases, and the temperature rise depends on the amount of soot collected as well as the temperature of the exhaust gas upstream of the DPF. These high temperature exhaust gases, which can approach 750° C. and higher, subsequently will pass over the SCR catalyst. Thus, there has been much emphasis recently on improving the thermal stability of the SCR catalyst, both for vanadia-based catalysts as well as Cu—, Fe— and other base-metal catalysts. It is generally accepted that the catalyst must be stable to temperatures of up to 800° C. for short periods of time. In order to test the durability of catalyst formulations, it is necessary to develop tests that simulate real-world exposure conditions. Ford researchers[1] have developed an accelerated aging protocol for SCR catalysts that simulates on-road conditions over a span of 120,000 miles. This test involves exposing the catalyst to a reactant gas stream that includes water (5 vol %), for a time of 64 hr at 670° C. at relatively high gas flow rate (gas hourly space velocity, GHSV=30,000 hr−1). These conditions of time and temperature are used as a reference point for the method of the present invention.
Concern over the volatility of vanadia at high temperatures, e.g., when the SCR catalyst is located downstream of the DPF, is thus an issue that may limit the available market for vanadia-based mobile SCR catalysts and is a key consideration in catalyst development. There has thus remained in the art a need to be able to evaluate the degree of vanadia volatilization from SCR catalysts. Further, there has remained a need in the art for a deNOx selective catalytic reduction catalyst system which demonstrates zero vanadia loss downstream thereof. It is an object of the invention to address these shortcomings of the prior art.